1. Field of the Invention
The present invention relates to circuit simulation and, more particularly, to a method and apparatus for simulating the time-varying electrical response of circuit devices and the time-varying temperature response of the circuit devices.
2. Description of Related Art
Today's circuit designers have to rely on circuit simulation to verify the functionality of a circuit design. Accurate device models are necessary for obtaining accurate simulation results.
During operation of devices, the power dissipation of each device will cause it to heat up whereupon its operating temperature will be higher than the environmental temperature. The difference in temperature for each device depends on its operating condition, which may vary during operation.
Device temperature has a substantial effect on device behavior and, therefore, on in-circuit simulation results. The effect of device temperature on the device behavior is called self-heating.
Heretofore, circuit simulation treated the temperature of each device as another freedom just like node voltages. For example, in a MOSFET, without self-heating effect, there are three independent states, namely, Vgs, Vds and Vbs. Once these states have been provided to a device model, a circuit simulator can process the device model and calculate the terminal currents and charges as well as the derivatives of those currents and charges with respect to node voltages.
With self-heating, in addition to Vgs, Vds and Vbs, there is another state, namely, temperature. Now, the device behavior (current, charges and their derivatives) are determined for these four states. Thus, calculation of the derivatives of all the currents and charges with respect to temperature during model evaluation is also required.
One problem associated with circuit simulation that includes modeling of self-heating is the need to derive the derivatives of all the outputs (currents/charges) and intermediate variables with respect to temperature. For many circuits determining the derivatives of all the outputs and intermediate variables with respect to temperature can be a large task. To this end, many circuit simulators include no self-heating support because the effort to calculate the derivatives of the outputs and their intermediate variables with respect to temperature is simply too big.
Another problem associated with circuit simulators that support modeling of self-heating, is that, due to bugs, or non-complete derivatives, the self-heating model is not well-developed and often causes convergence problems during simulation. This is a common problem for popular bipolar junction transistor circuit simulation models.
Still yet another problem is that the need to derive derivatives of all the outputs and intermediate variables with respect to temperature increases the amount of time required to perform a circuit simulation over a circuit simulator that does not support self-heating.
What is, therefore, needed, and not disclosed in the prior art, is a method and apparatus for performing a circuit simulation that overcomes the above problems and others.